1. Field of the Invention
The present invention relates in general to a vibration damping support apparatus for supporting a power unit of an automobile, and more particularly to such a support apparatus which positively controls vibrations that are transmitted from the power unit to the body of the automobile, to thereby provide excellent vibration damping and isolating effects with respect to input vibrations over a relatively wide frequency range, that is, from low-frequency vibrations such as shakes to high-frequency vibrations such as booming noise.
2. Discussion of the Related Art
To damp or isolate vibrations and noises in the interior of an automobile, a power unit including an engine is flexibly supported on the vehicle body by a plurality of engine mounts, so as to reduce transmission of vibrations from the power unit to the vehicle body. When the power unit is flexibly or elastically supported by conventional passive-type elastic mounts having predetermined spring or damping characteristics, however, vibrations are undesirably amplified around a resonance point of a suspension system that consists of the power unit and its support device (i.e., the elastic mounts). Thus, the conventional arrangement as described above may adversely increase the vibrations transmitted to the vehicle body.
As one type of engine mounts, there is also proposed a fluid-filled elastic mount whose spring or damping characteristics are variable. However, the above problem due to the resonance cannot be solved by use of such an elastic mount. Namely, since the vibrations transmitted from the power unit usually include vibratory components in a relatively wide frequency range, it is extremely difficult for the above type of the elastic mount to effectively damp or isolate the whole frequency range of the input vibrations.
In recent years, a vibration control device for positively reducing vibration of a subject has been developed, which device includes oscillating means for generating an oscillating force for eliminating or cancelling the vibration of the subject. An example of such device is disclosed in JP-A-3-219139, which includes an actuator interposed between the power unit and the vehicle body. The actuator is adaptively controlled based on a signal received from a vibration sensor attached to the power unit, and a signal received from a vibration sensor attached to the vehicle body, so as to generate the counter oscillating force which reduces the vibrations transmitted from the power unit to the vehicle body.
However, such a vibration control device as described above is not satisfactory in its ability to reduce the vibrations that are transmitted from the power unit to the vehicle body. Thus, it has been difficult for the known vibration control device to provide sufficiently high vibration damping and isolating effects.
In the adaptive control of the oscillating means by the positive-type vibration control device as described above, control signals for controlling the oscillating means are calculated based on the following information: (a) an error signal representative of the magnitude of vibrations to be controlled, which take place at a given monitoring point on the vehicle body; and (b) a reference signal based on which the adaptive control of the oscillating means is effected according to an algorithm. The selection of these two kinds of signals and the design and positioning of the oscillating means are important factors for achieving desirable adaptive control of the oscillating means. In particular, it is extremely important to assure that the obtained reference signal indicated above at (b) represents such information that has a close relationship with the characteristics of the vibrations to be reduced at the monitoring point of the vehicle body. The monitoring point is selected in an area where the vibration damping is desired.
In this connection, the vibrations of the power unit which cause the vehicle body to vibrate include (1) high-frequency vibrations, such as booming noise, due to explosion process of the engine and rotation of its crankshaft, and (2) low-frequency vibrations, such as shakes, due to the resonance of the power unit suspension system resulting from an oscillating force applied from the vehicle wheels while the automobile is running on a bumpy road surface. The high-frequency vibrations indicated above can be effectively isolated by an adaptive control using the signal from the vibration sensor attached to the power unit, since the high-frequency vibrations detected at virtially all points on the vehicle body are highly closely related to the vibrations of the power unit. On the other hand, when the low-frequency vibrations such as shakes arise from the resonance of the power unit suspension system, an adaptive control of the oscillating means on the basis of the vibration of the power unit will usually result in reduction in the oscillation of the power unit per se. To favorably control the low-frequency vibrations, however, it is undesirable to use the vibrations of the power unit as a control signal in a control circuit for the oscillating means, from the standpoints of the construction and control accuracy of the control circuit. Thus, using the vibrations of the power unit as the control signal makes it difficult to achieve sufficiently high vibration damping and isolating effects with respect to the low-frequency vibrations.